To date, numerous bioactive substances exhibiting diverse actions on various organs and cells have been isolated and identified, and their functions elucidated. The diverse bioactivity of these substances in various organs and cells is normally expressed via receptors to which they are bound; however, it remains unknown as to whether all organs and cells share the same receptors or if the organ and cell specificity of receptors is variable.
Steroid hormones produced by reproductive glands were assumed to be responsible for the regulation of gonadotropic hormone secretion from the pituitary gland. Then activin was first isolated as a regulator that promotes the secretion of follicle-stimulating hormone (FSH) from the anterior lobe of the pituitary gland. After the discovery of activin and inhibin, which antagonizes activin, activin and inhibin have drawn attention as facilitating a new mechanism of hormone secretion regulation for the hypothalamus-pituitary-reproductive organ system. Analysis of the bioactivity of activin has revealed various biological actions, including induction or inhibition of cell differentiation in the blood cell system and reproductive organs, and neuronal cell survival maintenance action, in addition to regulation of FSH secretion. However, much remains unknown as to activin's detailed mechanism of action.
The activin receptor gene has been cloned in humans, mice, rats and other animals and structural analysis has revealed two types: types I and II. It has also been shown that types I and II each have two subtypes, A and B, and that type IIB can be divided into four subtypes by alternative splicing. Receptor proteins assumed to mediate the diverse bioactivity of activin have varying affinity for different forms of activin; their expression is known to occur over a wide range without organ or cell specificity. Activin is known to induce or inhibit cell differentiation, but the expression of activin or its receptors has not been known to significally vary during all differentiation. The mechanism of diverse actions of activin remains to be elucidated in detail.